U.S. patent number 11,446,083 [Application Number 16/959,063] was granted by the patent office on 2022-09-20 for microwave amplifier.
This patent grant is currently assigned to CREO MEDICAL LIMITED. The grantee listed for this patent is Creo Medical Limited. Invention is credited to Christopher Duff, Christopher Paul Hancock.
United States Patent |
11,446,083 |
Hancock , et al. |
September 20, 2022 |
Microwave amplifier
Abstract
A microwave amplifier having a load network which provides more
efficient amplification of a low power microwave frequency signal.
The amplifier comprises a transistor and a load network coupled to
the transistor output to shape a waveform of an amplified microwave
signal at the transistor current source plane. The load network
comprises: a fundamental matching network to provide impedance
matching at a fundamental frequency; a half-wave transmission line
for a second harmonic frequency disposed between the transistor
output and the fundamental matching network; a quarter-wave stub
and a five-quarter-wave stub for a third harmonic frequency
arranged on the half-wave transmission line to provide an open
circuit condition at the third harmonic; and a quarter-wave stub
for the second harmonic frequency and a quarter-wave stub for the
fundamental frequency, arranged on the half-wave transmission line
to provide a short circuit condition at the second harmonic
frequency.
Inventors: |
Hancock; Christopher Paul
(Bath, GB), Duff; Christopher (Chepstow,
GB) |
Applicant: |
Name |
City |
State |
Country |
Type |
Creo Medical Limited |
Chepstow |
N/A |
GB |
|
|
Assignee: |
CREO MEDICAL LIMITED (Chepstow,
GB)
|
Family
ID: |
1000006572639 |
Appl.
No.: |
16/959,063 |
Filed: |
April 26, 2019 |
PCT
Filed: |
April 26, 2019 |
PCT No.: |
PCT/EP2019/060720 |
371(c)(1),(2),(4) Date: |
June 29, 2020 |
PCT
Pub. No.: |
WO2019/207098 |
PCT
Pub. Date: |
October 31, 2019 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20200397506 A1 |
Dec 24, 2020 |
|
Foreign Application Priority Data
|
|
|
|
|
Apr 27, 2018 [GB] |
|
|
1806940 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61B
18/1815 (20130101); H03F 3/19 (20130101); H03F
1/56 (20130101); A61B 2018/1823 (20130101); A61B
2018/1861 (20130101); H03F 2200/387 (20130101); H03F
2200/423 (20130101); A61B 2018/00577 (20130101); A61B
2018/00982 (20130101) |
Current International
Class: |
H03F
3/191 (20060101); H03F 3/19 (20060101); H03F
1/56 (20060101); A61B 18/18 (20060101); A61B
18/00 (20060101) |
Field of
Search: |
;330/302,305,306 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
2551339 |
|
Dec 2017 |
|
GB |
|
2013009031 |
|
Jan 2013 |
|
JP |
|
Other References
Fu, W., et al., "High efficiency GaN E power amplifier at 5.8GHz
with harmonic control network", IEEE Wireless Power Transfer
Conference (WPTC), ISBN 978-1-4799-2923-8, pp. 205-207, (2014).
cited by applicant .
International Search Report and the Written Opinion issued by the
International Searching Authority in corresponding International
Patent Application No. PCT/EP2019/060720, dated Aug. 16, 2019.
cited by applicant .
Raemer, A. et al., "Software optimization of a supply modulated
GaN-amplifier for baseband access ET systems", German, IEEE, ISBN:
978-1-4577-2096-3, pp. 1-4, Mar. 12, 2012. cited by applicant .
Search Report issued by the United Kingdom Patent Office in
corresponding British Patent Application No. 1806940.01, dated Oct.
23, 2018. cited by applicant.
|
Primary Examiner: Choe; Henry
Attorney, Agent or Firm: ArentFox Schiff LLP
Claims
The invention claimed is:
1. A microwave amplifier for amplifying electromagnetic (EM)
signals at a fundamental frequency, the amplifier comprising: a
transistor configured to provide an amplified microwave signal at
an output thereof; and a load network coupled to the output for
shaping a waveform of the amplified microwave signal, wherein the
load network comprises: a fundamental matching network that is
tunable to provide impedance matching at the fundamental frequency;
a half-wave transmission line for a second harmonic frequency of
the amplified microwave signal, the half-wave transmission line
being disposed between the output and the fundamental matching
network; a quarter-wave stub and a five-quarter-wave stub for a
third harmonic frequency of the amplified microwave signal arranged
on the half-wave transmission line to provide an open circuit
condition at the third harmonic frequency; and a quarter-wave stub
for the second harmonic frequency and a quarter-wave stub for the
fundamental frequency, arranged on the half-wave transmission line
to provide a short circuit condition at the second harmonic
frequency.
2. A microwave amplifier according to claim 1, wherein the
quarter-wave stub and the five-quarter wave stub for the third
harmonic frequency are arranged to oppose each other at a distance
along the half-wave transmission line equal to a quarter-wave for a
third harmonic frequency.
3. A microwave amplifier according to claim 1, wherein the
quarter-wave stub for the second harmonic frequency and the
quarter-wave stub for the fundamental frequency are arranged to
oppose each other at an output of the half-wave transmission
line.
4. A microwave amplifier according to claim 1, wherein a bias
voltage is applied to the transistor through the quarter-wave stub
for the fundamental frequency.
5. A microwave amplifier according to claim 4, further comprising a
capacitor arranged between the bias voltage input and the
quarter-wave stub for the fundamental frequency.
6. A microwave amplifier according to claim 1, wherein the
half-wave transmission line for the second harmonic frequency
comprises a quarter-wave transmission line for a third harmonic
frequency, the quarter-wave stub and five-quarter-wave stub for the
third harmonic frequency being arranged to oppose each other at the
output of the quarter-wave transmission line for the third harmonic
frequency.
7. A microwave amplifier according to claim 1, wherein the
transistor is a GaN-based HEMT.
8. A microwave signal generator for generating high power microwave
electromagnetic (EM) radiation, the generator comprising: a
microwave source arranged to generate microwave EM radiation at a
first power, and a microwave amplifier according to claim 1,
wherein the microwave amplifier is arranged to amplify the
microwave EM radiation from the first power to a second power that
is higher than the first power.
9. A microwave signal generator according to claim 8, further
comprising a direct current (DC) power source for supplying DC
energy.
10. An electrosurgical apparatus for performing electrosurgery, the
apparatus comprising: a microwave source arranged to generate
microwave electromagnetic (EM) radiation at a first power; a
microwave amplifier according to any claim 1, arranged to amplify
the microwave EM radiation from a first power to a second power
that is higher than the first power; a probe arranged to deliver
the microwave EM radiation at the second power from a distal end
thereof for treating biological tissue; and a feed structure for
conveying microwave EM energy from the microwave generator to the
microwave amplifier and to the probe, wherein the probe is arranged
at a distal end of the feed structure.
11. An electrosurgical apparatus according to claim 10, further
comprising a direct current (DC) power source for supplying DC
energy to the microwave signal generator, wherein the DC power
source is integrated with the probe.
12. An electrosurgical apparatus according to claim 10, wherein the
microwave amplifier is mounted in the probe.
13. An electrosurgical apparatus according to claim 10, wherein the
microwave signal generator is mounted in the probe.
14. An electrosurgical apparatus according to claim 10, wherein the
apparatus further comprises a scoping device having a body and an
instrument cord, wherein an instrument channel extends through the
instrument cord, and wherein the probe is insertable through the
instrument channel.
15. An electrosurgical apparatus according to claim 10, wherein the
apparatus further comprises a handle connected to the probe via a
flexible shaft.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a National Stage entry of International
Application No. PCT/EP2019/060720, filed on Apr. 26, 2019, which
claims priority to British Patent Application No. 1806940.1, filed
on Apr. 27, 2018. The disclosures of the priority applications are
hereby incorporated in their entirety by reference.
FIELD OF THE INVENTION
The invention relates to microwave amplifiers. In particular it
relates to a microwave amplifier configuration for use with an
electrosurgical device for treating biological tissue with
microwave energy.
BACKGROUND TO THE INVENTION
The use of microwave energy in the treatment of biological tissue
through electrosurgery is well known. However, it remains a
challenge to deliver microwave energy in a closely controlled
manner, primarily due to the effect of losses between the microwave
source and an applicator structure which is in contact with the
biological tissue to be treated. These effects can be particularly
problematic in minimally invasive procedures which make use of
surgical scoping devices such as endoscopes. Surgical scoping
devices typically comprise a body from which an instrument cord
extends. An applicator structure is inserted into a patient's body
through an instrument channel which is a lumen extending through
the length of the instrument cord. Delivering microwave energy to
the applicator therefore requires transmitting this energy through
the instrument cord.
In order to be able to treat biological tissue, large amounts of
energy need to be delivered by the applicator. This means that high
power signals must be transmitted through the instrument cord.
However, transmitting high power signals results in large losses,
which can lead to undesirable endoluminal heating which can have a
negative effect on the body. Overcoming these issues usually
requires that lower power signals are transmitted through the
instrument cord, which leads to longer treatment times. Longer
treatment times reduces patient comfort and may also prolong
recovery time after surgery.
SUMMARY OF THE INVENTION
At its most general, the present invention is a microwave amplifier
having a load network which provides more efficient amplification
of a low power microwave frequency signal. The microwave amplifier
of the present invention is particularly suited for use with an
electrosurgical apparatus for the treatment of biological tissue,
for example ablation, resection, coagulation etc.
The increased efficiency resulting from an output load network
according to the present invention allows the microwave amplifier
and/or generator to be located at any point between a DC power
source and a microwave applicator structure for delivering energy
to tissue. A smaller, more efficient amplifier has lower power
requirements and also a reduced need for cooling. For example, in
some embodiments the amplifier and/or microwave generator may be
incorporated into a handle of an electrosurgical apparatus, or
within the applicator structure itself. The present invention also
allows the manufacture of a portable generator unit for use with an
electrosurgical apparatus.
According to a first aspect of the present invention, there is
provided microwave amplifier for amplifying electromagnetic (EM)
signals at a fundamental frequency, the amplifier comprising: a
transistor configured to provide an amplified microwave signal at
an output thereof; and a load network coupled to the output for
shaping a waveform of the amplified microwave signal at the
transistor current source plane, wherein the load network
comprises: a fundamental matching network that is tunable to
provide impedance matching at the fundamental frequency; a
half-wave transmission line for a second harmonic frequency of the
amplified microwave signal, the half-wave transmission line being
disposed between the output and the fundamental matching network; a
quarter-wave stub and a five-quarter-wave stub for a third harmonic
frequency of the amplified microwave signal arranged on the
half-wave transmission line to provide an open circuit condition at
the third harmonic frequency; and a quarter-wave stub for the
second harmonic frequency and a quarter-wave stub for the
fundamental frequency, arranged on the half-wave transmission line
to provide a short circuit condition at the second harmonic
frequency. For example, the amplifier may be an integrated circuit
based amplifier.
With this configuration, the fundamental matching network can
operate independently of the waveform shaping effect provided by
the rest of the load network. Put another way, the stubs that
provide the waveform shaping effect are configured in the invention
to counteract or inhibit any effect on the fundamental frequency to
which the fundamental matching network is matched. This may allow
the fundamental matching network to be pre-configured, e.g. before
connection to the transistor. On assembling the amplifier, the load
network can be optimised (e.g. tuned) to achieve the required
waveform shaping effect through appropriate placement of the stubs
without affecting the impedance match at the fundamental frequency
provided by the fundamental matching network.
This independence is achieved in particular through the use of the
five-quarter-wave stub for the third harmonic frequency. This stub
removes the effect of the quarter-wave stub for the third harmonic
frequency on the fundamental and second harmonic matching.
Independent tuning is particularly useful as amplifier transistors
are commonly supplied as part of a package, and not as individual
components. Tuning of the load network can be highly dependent on
the characteristics of the package, but these characteristics are
rarely given by manufacturers or suppliers. As a result, it is
often necessary to tune the load network for a particular
transistor package. In the present invention, this can be done
without affecting the impedance match of the load network at the
fundamental frequency, making it possible to design and tune part
of the load and matching network before precise characteristics of
a transistor package are known. Furthermore, if the possible impact
of the harmonic tuning network, i.e. the waveform shaping part of
the load network, upon the stability of the amplifier is unknown
due to a lack of transistor information, a less efficient class of
amplifier (for example class B) may be designed and built before
characteristics of the package are known, and a higher efficiency
amplifier, with reduced conduction angle modes, such as class F,
produced and tested experimentally `on the bench` with the
transistor package in place.
The amplifier of the present invention is a class F microwave
amplifier. By terminating odd harmonics in open circuit
terminations and terminating even harmonics in short circuit
terminations, high amplifier efficiencies are achieved with a
square wave voltage waveform and half sinusoidal wave current
waveform at the transistor current source plane, or output. In
particular it has been found that the present invention provides
high efficiencies of at least 80% while considering only up to the
third harmonic of the fundamental frequency in the load network.
This level of efficiency is sufficient for use in a microwave
generator line-up of an electrosurgical apparatus as described
below. In some embodiments, the load network may comprise
additional terminations for higher-order harmonic frequencies in
order to achieve higher efficiencies. Theoretically, efficiencies
approaching 100% can be achieved if a sufficient number of
higher-order harmonics are terminated by the load network.
Preferably the quarter-wave stub and the five-quarter wave stub for
the third harmonic frequency are arranged to oppose each other at a
distance along the half-wave transmission line equal to a
quarter-wave from the transistor current source plane for a third
harmonic frequency. This ensures proper open circuit termination of
the third harmonic frequency. The ability to tune the load network
independently of a fundamental matching network accounts for the
unknown electrical length between the intrinsic transistor current
source plane and the package external plane, that is, the
electrical distance between the transistor output and the output of
the package within which the transistor is provided. In some
embodiments, the half-wave transmission line for the second
harmonic frequency comprises a quarter-wave transmission line for a
third harmonic frequency (including the internal package drain
connection electrical length), and so the quarter-wave stub and
five-quarter-wave stub for the third harmonic frequency may be
arranged to oppose each other at the output of the quarter-wave
transmission line for the third harmonic frequency.
Preferably, the quarter-wave stub for the second harmonic frequency
and the quarter-wave stub for the fundamental frequency are
arranged to oppose each other at an output of the half-wave
transmission line. This ensures proper closed circuit termination
of the second harmonic frequency.
Optionally, a bias voltage may be applied to the transistor through
the quarter-wave stub for the fundamental frequency. Preferably a
shunt capacitor to ground is also arranged at the connection of the
bias voltage input and the quarter-wave stub for the fundamental
frequency. The capacitor may provide a sufficiently low reactance
to approximate a short circuit at microwave frequencies.
According to a second aspect of the present invention, there is
provided a microwave signal generator for generating high power
microwave electromagnetic (EM) radiation, the generator comprising:
a microwave generator arranged to generate microwave EM radiation
at a first power, and a microwave amplifier which may be an
amplifier according to the first aspect of the present invention.
The microwave amplifier is arranged to amplify the microwave EM
radiation from the first power to a second power that is higher
than the first power. By using a microwave amplifier as described
above, the present invention allows the manufacture of easily
portable microwave signal generators which are capable of producing
high power microwave EM radiation. The high efficiency apparatus
may be smaller and have reduced power and cooling requirements. A
portable generator may be desirable, for example, for use with an
electrosurgical haemostatic device, especially a device which may
be used in emergency situations. The microwave signal generator may
comprise a direct current (DC) power source for supplying DC
energy, which may be required by the microwave generator. The DC
power supply may be in the form of a battery, in particular a
removable battery. In this way, a portable generator may be
provided which provides sufficient energy for haemostasis and
coagulation in which the power supply can easily be replaced if
further energy delivery is required.
According to a third aspect of the present invention, there is
provided an electrosurgical apparatus for performing
electrosurgery, the apparatus comprising: a microwave signal
generator arranged to generate microwave electromagnetic (EM)
radiation at a first power; a microwave amplifier according to the
first aspect of the invention, arranged to amplify the microwave EM
radiation from a first power to a second power that is higher than
the first power; a probe arranged to deliver the microwave EM
radiation at the second power from a distal end thereof for
treating biological tissue; and a feed structure for conveying
microwave EM energy; wherein the probe is arranged at a distal end
of the feed structure, and the microwave signal generator and the
microwave amplifier are distributed along the feed structure.
By providing an electrosurgical apparatus in this way, using a
microwave amplifier as described above with respect to the first
aspect, high power microwaves for electrosurgery can be produced
while reducing losses throughout the feed structure and avoiding
problems which stem from endoluminal heating.
The present invention allows the microwave amplifier to be located
closer to, or even integrated with, the probe, reducing losses
normally arising through transmission of high power microwave EM
energy to the probe. This has numerous advantages, such as allowing
reduced diameter cables to be used, in turn allowing electrosurgery
in places which would otherwise be difficult to reach. Reduced
losses also means reduced heating of a transmission cable forming
the feed structure.
The present invention also ensures reduced power requirements for
the amplifier, so there may also be reduced losses and power
dissipation throughout the feed structure leading to the microwave
amplifier.
In some embodiments, the microwave signal generator may also be
integrated with the probe. Microwave power losses and associated
drawbacks present in known devices, as described above, can
therefore be further avoided or reduced. The apparatus may further
comprise a direct current (DC) power source for supplying DC energy
to the microwave signal generator, wherein the DC power source is
also integrated with the probe. In this way, microwave generation
may be carried out entirely within the probe, and in some
embodiments no external power source is required.
In some embodiments, the electrosurgical apparatus may comprise a
scoping device having a body and an instrument cord, wherein an
instrument channel extends through the instrument cord and the
probe is insertable through the instrument channel. For example,
the scoping device may be an endoscope, gastroscope, laparoscope or
the like. The microwave signal generator may be integrated with the
body of the scoping device in order to provide a portable
electrosurgical apparatus having the advantages of the present
invention. In some embodiments, a DC power source may be integrated
with the body of the scoping device.
Optionally, the electrosurgical apparatus may comprise a handle,
which may be connected to the probe via a flexible shaft.
Preferably, the flexible shaft is insertable through the instrument
channel of a scoping device. The microwave signal generator may be
integrated with the handle. In some embodiments, a DC power source
may be integrated with the handle.
In this specification "microwave" may be used broadly to indicate a
frequency range of 400 MHz to 100 GHz, but preferably the range 1
GHz to 60 GHz. Specific frequencies that have been considered are:
915 MHz, 2.45 GHz, 3.3 GHz, 5.8 GHz, 10 GHz, 14.5 GHz and 24
GHz.
Similarly, references to a "conductor" or "conductive" material
herein are to be interpreted as meaning electrically conductive
unless the context makes clear that another meaning is
intended.
BRIEF DESCRIPTION OF THE DRAWINGS
Examples embodying the invention are discussed in detail below with
reference to the accompanying drawings, in which:
FIG. 1 is a schematic view of a complete electrosurgical apparatus
in which the present invention is applied;
FIG. 2 is a schematic view of a microwave generator line up;
FIG. 3 is a schematic view of components in an output stage which
may be used with the present invention;
FIG. 4 shows a prior art load network;
FIG. 5 shows a load network in accordance with the present
invention;
FIG. 6 shows a graph of output voltage and current for an amplifier
according to the present invention.
DETAILED DESCRIPTION
FIG. 1 is a schematic diagram of a complete electrosurgical
apparatus 100 in which the present invention may be used.
The apparatus comprises a surgical scoping device 114, such as an
endoscope, gastroscope, laparoscope or the like. The surgical
scoping device 114 comprises a body 116 having a number of input
ports and an output port from which an instrument cord 120 extends.
The instrument cord 120 comprises an outer jacket which surrounds a
plurality of lumens. The plurality of lumens convey various things
from the body 116 to a distal end of the instrument cord 120. One
of the plurality of lumens is an instrument (working) channel. A
flexible shaft 112 is insertable along the entire length of the
instrument (working) channel. Other lumens may include a channel
for conveying optical radiation, e.g. to provide illumination at
the distal end or to gather images from the distal end. The body
116 may include an eye piece 122 for viewing the distal end. In
order to provide illumination at the distal end, a light source 124
(e.g. LED or the like) may be connected to the body 116 by an
illumination input port 126.
At a proximal end of the flexible shaft 112 there is a handle 106,
which may be connected to receive a fluid supply 107 from a fluid
delivery device 108, such as a syringe, although this need not be
essential. If needed, the handle 106 can house an instrument
control mechanism that is operable by sliding a trigger 110, e.g.
to control longitudinal (back and forth) movement of one or more
control wires or push rods (not shown). If there is a plurality of
control wires, there may be multiple sliding triggers on the handle
to provide full control.
The apparatus 100 may also comprise a generator 102 for supplying
microwave frequency and, optionally, radiofrequency (RF)
electromagnetic (EM) energy to a distal assembly 118. In some
embodiments, the generator 102 is configured as a DC power source
to supply only DC energy. The generator 102 is connected to the
handle 106 by an interface cable.
At a distal end of the flexible shaft 112, there is a distal end
assembly, or applicator, 118 (not drawn to scale in FIG. 1) that is
shaped to pass through the instrument channel of the surgical
scoping device 114 and protrude (e.g. inside the patient) at the
distal end of the instrument cord 120. The distal end assembly
includes an active tip for delivering microwave energy into
biological tissue, as discussed in more detail below.
The structure of the distal assembly 118 may be arranged to have a
maximum outer diameter equal to or less than 2.0 mm, e.g. less than
1.9 mm (and more preferably less than 1.5 mm) and the length of the
flexible shaft can be equal to or greater than 1.2 m.
In some embodiments, the body 116 may include a DC power source 128
that is connected to delivery DC energy to the distal end assembly
118 along the flexible shaft, e.g. using suitable leads. In other
embodiments, the DC power source may be provided in place of the
generator 102. The DC power source 128 or 102 may be a battery
(e.g. a lithium ion battery), supercapacitor or a fuel cell, which
may be mounted in the body 116. In another example, the DC power
source 128 or 102 may be a coupling unit arranged to inductively or
magnetically couple energy into the device from a remote source
(not shown). In this case, the coupling unit may comprise internal
rectification and filtering to obtain a DC signal from coupled
energy.
In yet further examples, the DC power source may be part of the
distal end assembly 118, in which case leads extending along the
instrument channel are not required.
It may be desirable to control the position of at least the distal
end of the instrument cord 120. The body 116 may include a control
actuator 130 that is mechanically coupled to the distal end of the
instrument cord 120 by one or more control wires (not shown), which
extend through the instrument cord 120. The control wires may
travel within the instrument channel or within their own dedicated
channels. The control actuator 130 may be a lever or rotatable
knob, or any other known catheter manipulation device. The
manipulation of the instrument cord 120 may be software-assisted,
e.g. using a virtual three-dimensional map assembled from computer
tomography (CT) images.
FIG. 2 is a schematic view showing components of a microwave
generator line up 131. The microwave generator line up 131 includes
generator circuitry 132 for producing a low power microwave signal,
and an output stage 134 for amplifying the signal to a level
suitable for electrosurgery, e.g. ablation treatment of biological
tissue.
The generator circuitry 132 comprises an oscillator 144 for
outputting a microwave signal, e.g. having a frequency of 1 GHz or
more, preferably 5.8 GHz or more. The oscillator 144 may be a
voltage controlled oscillator (VCO) or a dielectric resonator
oscillator (DRO). The oscillator 144 may receive DC power as an
input. DC power may be provided by the generator 102 or by the DC
power source 128. The output from the oscillator 144 may be pulsed
by a modulator 146. The output from the oscillator 144 is provided
to a driver amplifier 148, which is arranged to generate an input
signal for the output stage 134. The driver amplifier 148 may be
any suitable MMIC device. The line up 131 may further include an
attenuator (not shown) to provide control over the amplitude of the
signal delivered to the output stage 134. The output stage 134
itself may comprise a biasing circuit 150 and a GaN-based
transistor 152 configured as a power amplifier. The output stage
may include circuitry (not shown) to protect the output stage
components from signal reflects back from the radiating structure.
For example, a circulator may be mounted on a forward path from the
GaN-based transistor. The circulator may divert reflected power to
a dump load. However, this protection structure is not essential
because GaN-based structures can be robust enough to cope. The
output stage 134 also includes a load network, as described
below.
Components of the microwave generator line up 131 may be positioned
within different parts of the electrosurgical apparatus 100. In
some embodiments, the generator line up 131, including both the
generator circuitry 132 and output stage 134, may form part of a
microwave generator 102. By using a microwave amplifier according
to the present invention, the microwave generator 102 may be easily
portable. Alternatively, the oscillator 144 and modulation switch
146 may be part of the distal end assembly 118, which may be
desirable to significantly reduce losses associated with passing
microwave signals through cables. Optionally, the oscillator 144
and modulation switch 146 may be located in or at the body 116 of
the surgical scoping device, and the output stage located in the
applicator 118, reducing losses as only low-power microwave signals
need to be transmitted along the instrument channel. In another
example, the whole generator circuitry 132 (i.e. including the
driver amplifier 148) may be located at a proximal distance from
the distal end assembly, e.g. in the body 116. Thus, the input
signal for the output stage 134 may be transmitted along the
instrument channel.
To illustrate, one example may comprise a DRO with an output power
of 10 dBm (10 mW) and a MMIC with a gain of 20 dB located in the
body of the scoping device. Even if the insertion loss of the cable
is 10 dB in this scenario, there would still be 20 dBm (100 mW)
available at the distal end assembly. In this example, the output
stage may comprise a second MMIC followed by the GaN-based
transistor 152. If the second MMIC has a gain of 10 dB and a high
density GaN device a gain of 10 dB, then there will be 40 dBm (10
W) available for delivery.
The transmission line 136 may be any suitable structure for
conveying the microwave power generated by the output stage 134 to
the radiating structure. For example, both coaxial (including
waveguide) structures and microstrip structures may be used, as
explained in more detail below.
FIG. 3 is a schematic diagram of the components in an output stage
134 that can be used in an embodiment of the invention. The output
stage 134 uses a high density GaN-based HEMT as an amplifier for an
input received from the generator circuitry 132. Whilst any
suitable amplifier configuration may be used, in accordance with
the present invention it is most desirable to bias the output
transistor using a class F structure. This configuration allows the
device to take the power added efficiency (PAE) close to its
theoretical limit. In particular, the structure shown in FIG. 3 may
be able to achieve a PAE of at least 80%, or up to 90%. It is these
high efficiencies resulting from the form of the output stage 134
which allows components of the microwave generator line up 131 to
be separated and spread across components of the electrosurgical
apparatus 100, as only lower power microwave signals are required
to be sent to the output stage 134, resulting in smaller losses
when the signal is passed through cables. High efficiencies also
allow for construction of a microwave generator 102 which is
portable.
The class F structure in FIG. 3 provides a load network at an
output of the HEMT 152 amplifier, the load network comprising a
matching circuit 188 and a resonant circuit 190. A first resonant
circuit (e.g. a LC or tank circuit) 184 is also provided at an
input to the GaN-based HEMT 152 with a respective matching circuit
186 (e.g. a series LC circuit). The load network, made of the
output resonant circuit 188 and matching network 190 together, is a
harmonic termination network, which is explained below. The device
is biased near or at cut-off, in a similar manner to class B
operation.
In order to increase the efficiency in terms of the amount of
microwave power produced at the output to DC and input microwave
signal at the input, it is desirable to operate the GaN device
using a scheme other than the standard linear Class A scheme, i.e.
Class B, AB, C, D, E or F.
The efficiency of an amplifier is limited by the characteristics of
the transistors used in the design. If class F design is used then
it is theoretically possible to achieve 100% efficiency, but this
assumes that the transistor is an ideal current source. In
practice, it should be possible to achieve at least 70% power added
efficiency (PAE) using a class F arrangement.
A class F amplifier has as its base a class B amplifier, with the
component transistor being biased between the amplifier's knee and
transconductance regions rather than purely in the transconductance
region. This biasing results in clipping of the current and voltage
output waveforms, i.e. the sinusoid output waveforms are distorted,
and waveform engineering can be performed by selecting an
appropriate load or harmonic termination network for the output of
the amplifying transistor.
For example, the second resonant circuit 190 may be configured to
shape the output waveform based on the load appearing as a short
circuit to even harmonics (i.e. short circuit at 2f.sub.1, where
f.sub.1 is the fundamental resonant frequency of the circuit) and
as an open circuit to odd harmonics (i.e. open circuit at
3f.sub.1). Accordingly, the drain voltage waveform is shaped
towards a square wave whereas the drain current is shaped such that
it resembles a half-wave sinusoidal waveform, dependent upon the
number of harmonics controlled. Note that for the n.sup.th
harmonic, f.sub.n=nf.sub.1 and .lamda..sub.n=.lamda..sub.1/n.
Higher-order harmonics can be accounted for, but result in
diminishing returns in terms of PAE. A resonant circuit which
accounts for the second and third harmonics is sufficient to
achieve at least 80% efficiency, and so represents a good balance
of efficiency and load network complexity/cost. By accounting for
only the second and third harmonics, the load network may be made
small enough to be provided as part of an integrated circuit. For
example, an integrated circuit based amplifier may be integrated in
the probe itself.
The first resonant circuit 184 assists in ensuring that the device
is driven by square wave pulses. The first resonant circuit 184 may
thereby introduce harmonic generation and allow simpler current
sources to be used. In some embodiments, the first resonant circuit
184 is not required and an input waveform is sinusoidal.
An example of a known load or harmonic termination network 200 for
the output of a transistor 202 is shown in FIG. 4. The load network
200 comprises a half-wave transmission line for the second harmonic
frequency (i.e. .lamda..sub.2/2 transmission line), formed from a
quarter-wave transmission line 204 for the third harmonic
frequency--a .lamda..sub.3/4 transmission line 204--and a
.lamda..sub.2/2-.lamda..sub.3/4 transmission line 212 connected in
series. The .lamda..sub.3/4 transmission line 204 length includes
the transmission line internal to the package of the transistor
leading to the drain output connection, the characteristics of
which may be unknown.
A quarter-wave stub 206 for the third harmonic frequency (a
.lamda..sub.3/4 stub) is arranged at the output of the quarter-wave
transmission line 204 in order to provide an open circuit to the
intrinsic transistor drain at the third harmonic frequency. Due to
the relationship between the harmonic and resonant frequencies, it
should be noted that .lamda..sub.3/4=.lamda..sub.1/12, and so the
quarter-wave stub for the third harmonic frequency may also be
considered a .lamda..sub.1/12 stub.
To provide a short circuit at the second harmonic frequency
f.sub.2, the load network 200 comprises a quarter-wave stub 208 for
the second harmonic frequency (a .lamda..sub.2/4 stub), arranged
opposite a quarter-wave stub 210 for the fundamental frequency (a
.lamda..sub.1/4 stub). These are arranged at the output of the
effective half-wave transmission line for the second harmonic.
A bias voltage, V.sub.dd, of the transistor is applied through the
quarter-wave stub 210 for the fundamental frequency. This ensures
that the bias feed is spaced a half-wavelength distance at the
second harmonic frequency from the transistor 202 in order to
provide the correct impedance at the second harmonic.
A sub matching network 214 is also provided, and can be tuned to
provide impedance matching at the fundamental frequency, f.sub.1,
while taking the rest of the circuit 200 into account. The sub
matching network 214, similarly to the rest of the load network
200, may comprise a further arrangement of transmission lines and
stubs, and a DC blocking capacitor may also be present.
However, tuning of the load network 200 to increase the efficiency
of the amplifier affects the requirements for the sub matching
network 214. Designing and tuning a sub matching network 214 which
is also adversely affected by the rest of the load network can be
difficult and time consuming, and may lead to sub-optimal results.
A network in accordance with the present invention overcomes these
difficulties, as explained below.
FIG. 5 shows a schematic diagram of a load network 300 in
accordance with the present invention.
The load network 300 is coupled to the output of a transistor 302
which is arranged to amplify microwave signals delivered to the
transistor 302 at a fundamental frequency, f.sub.1. The load
network 300 comprises a half-wave transmission line for the second
harmonic frequency, formed from a quarter-wave transmission line
304 for the third harmonic frequency (a .lamda..sub.3/4
transmission line) and a .lamda..sub.2/2-.lamda..sub.3/4
transmission line 310 connected in series. The .lamda..sub.3/4
transmission line 304 length includes the transmission line
internal to the package of the transistor leading to the drain
output connection, the characteristics of which may be unknown.
A quarter-wave stub 306 and a five-quarter-wave stub 308 for the
third harmonic frequency (a .lamda..sub.3/4 stub 306 and a
5.lamda..sub.3/4 stub 308, respectively) are arranged opposite each
other on the effective half-wave transmission line. They are
positioned away from the transistor 302, specifically the intrinsic
transistor 302 current source, at a distance equal to a
quarter-wave for a third harmonic frequency, i.e. at the output of
the quarter-wave transmission line 304. The quarter-wave stub 306
provides an open circuit at the third harmonic frequency, while the
five-quarter-wave stub 308 reinforces the open circuit at the third
harmonic frequency, while also counteracting the effect the
quarter-wave stub 306 has on the load network 300 at the second
harmonic and fundamental frequencies.
At the output of the effective half-wave transmission line, i.e.
the output of the .lamda..sub.2/2-.lamda..sub.3/4 transmission line
310, are arranged a quarter-wave stub 314 at the second harmonic
frequency and a quarter-wave stub 312 at the fundamental frequency.
These stubs provide a short circuit at the second harmonic
frequency.
By providing a short circuit at the second harmonic frequency and
an open circuit at the third harmonic frequency, the load network
300 produces an approximately square wave voltage output and a
half-sinusoid current output, as shown in FIG. 6 as a function of
time. This ensures that the amplifier operates at a high efficiency
of at least 80%.
A bias voltage, V.sub.dd, of the transistor is applied through the
quarter-wave stub 312 for the fundamental frequency. The
quarter-wave stub 312 in combination with the microwave capacitor
C.sub.bypass presents an open circuit at the fundamental frequency,
and so have no effect on the rest of the network 300. At the second
harmonic frequency, the quarter-wave stub 312 and capacitor present
a short circuit, reinforcing the effect of the quarter-wave stub
314 at the second harmonic frequency.
Transistors are typically available only as part of a package and
not as individual components. Information regarding the package
itself is usually limited, which introduces difficulties in
designing a load network. For example, it is often necessary to
know the exact distance between the transistor output, i.e. the
intrinsic current generator plane, and other components to form
transmission lines of the correct length. For this reason, it is
valuable to be able to tune a load network for an amplifier with
the transistor package in place, rather than relying only on a
hypothetical model.
The load network of the present invention allows tuning of the
matching network 316 and of the remainder of load network 300 to be
carried out independently. The fundamental matching network 316 can
be tuned to match impedance at the fundamental frequency without
being affected by tuning of the remainder of the load network. This
is due to the addition and positioning of the five-quarter-wave
stub 308 for the third harmonic frequency, which removes the effect
of quarter-wave stub 306 on the fundamental and second harmonic
matching while also reinforcing an open circuit for the third
harmonic frequency.
In this way, the intermediate portion of the load network 300 and
fundamental matching network 316 can in combination enable the
device to operate as a Class F amplifier, in which the tuning to
match to the relevant harmonics can be performed by the
intermediate portion independently of the tuning to the fundamental
performed by the fundamental matching network 316.
The fundamental matching network 316 may be designed and tuned for
a specific transistor 302 or transistor package. This can be done
in advance, and then mounted in the load network 300 configuration
of the invention. The intermediate portion can then be tuned to
enable the Class F operation without affecting the tuning of the
fundamental matching network.
The fundamental matching network 316 may comprise a further
arrangement of transmission lines and stubs, and a DC blocking
capacitor may also be present. The fundamental matching network 316
may be optimised for matching at the fundamental frequency during a
design phase using a model of the transistor 302, taking into
account the effective half-wave transmission line for the second
harmonic.
By using a microwave amplifier according to the present invention,
very high amplifier efficiencies can be achieved. As a result of
these high efficiencies a microwave generator for an
electrosurgical device can be made which is smaller and more
readily portable than known generators.
In addition, some embodiments of the present invention envisage
that the microwave generator or microwave amplifier may be located
within another section of the electrosurgical apparatus, such as
within a handle or a radiating structure. In these embodiments,
high amplifier efficiencies mean that DC or microwave frequency
signals can be transmitted to the microwave generator or amplifier
at a lower power. This results in less power dissipation, and makes
cooling of the apparatus easier to implement.
* * * * *